Detection of the 67.9 and 78.4 keV lines associated with the radioactive decay of Ti-44 in Cassiopeia A

We report the detection of the Sc-44 nuclear decay lines at 67.9 and 78.4 k eV associated with the nuclear decay of Ti-44 in Cassiopeia A. The line emi ssion was observed by the Phoswich Detection System instrument on board Bep poSAX, which recently observed the supernova remnant for over 500 ks. The d etection of the line emission with a flux of (2.1 +/-0.7) x 10(-5) photons cm(-2) s(-1) in each line (90% confidence) is at the 5 sigma significance l evel, if we can assume that the 12-300 keV continuum is adequately represen ted by a single power law. However, as the nature of the continuum is not c lear, we investigate various other possibilities. A more conservative estim ate of the line flux is made by assuming that a power-law continuum is at l east a good approximation to the continuum emission for a narrower 30-100 k eV energy range. With this limitation, the measured line flux is (1.9 +/-0. 9) x 10(-5) photons cm(-2) s-(1,) with the detection still at the 3. 4 sigm a significance level. We suggest that together with the Compton Gamma Ray O bservatory/COMPTEL measurement of the Ca-44 line at 1157 keV of (3.3 +/-0.6 ) x 10(-5) photons cm(-2) s(-1), a flux for all three lines of (2.5 +/-1.0) x 10(-5) photons cm(-2) s(-1) for Cas A can be adopted. This implies an in itial Ti-44 mass of (0.8-2.5) x 10(-4) M-circle dot.